The present invention relates to a process for regenerating the phase of synchronizing signals used in an apparatus for the optical transcription of data on a data carrier, during the write and/or read phases, particularly of digital data recorded on a disk. It also relates to an optical device for performing this process.
Recording methods are well known to the skilled Expert and fall outside the scope of the present invention. Information is generally recorded in the form of microreliefs along a track having a spiral or concentric circle configuration, the latter being the most frequently used in the case of recording digital data. It in particular facilitates random access to recorded data, as well as a recording subdivided into blocks or sectors.
During reading, no matter what the recording method, it is necessary to have signals permitting the synchronization of said reading. For this purpose, once again numerous methods are known.
According to a first method, in so-called multitrack systems, with each track reserved for the recording of useful information is associated at least one other track along which are recorded various informations and in particular clock signals permitting the synchronization of the reading of the useful information. According to a variant, the tracks used for synchronization are preetched and have optically detectable, regularly spaced disturbances. During reading, in a first variant (multibeam system), a first beam is focused on the useful information track and is used for reading such information, whilst a second separated beam, but constantly mechanically coupled with the first, is used for reading the synchronization information. In a second variant, (monobeam system) a single beam reads both informations. In this case, it is necessary for both information types to be easily discriminated. For example, the frequency spectra associated with these informations can differ. In this variant, the preetched synchronization information can also be used during the writing phase.
In order to increase the possible recording density, it has also been proposed to use a single track. In this case, the synchronizing clock signals can be derived from the actual information reading. To make synchronization easier, it is conventional practice to use so-called autosynchronizing codes or a maximum of transitions, no matter what the content of the source information to be recorded. Thus, in practice, the microreliefs have two reference levels associated respectively with logic values 0 and 1. The synchronizing signals are derived from the detection of the transition from a given level to another level. However, this type of coding does not permit a maximum recording density. It is also known for the purpose of increasing this density, to use non-autosynchronizing codes, e.g. the N.R.Z code (non-return to zero). The feature of this type of code is that there is no transition from one information bit to the other, if these two bits remain at the same logic value. It is then more difficult to derive the signals necessary for synchronization from the reading of the thus coded information. A solution can be found to this problem by regularly or non-regularly arranging along the tracks, recorded elements used solely for synchronization purposes called flags. Naturally, these flags must be "transparent" to the electronic circuits for detecting and processing the useful information. These synchronizing samples are used for resynchronizing for each passage of a flag beneath a read head the synchronizing signal generating circuits. It is therefore extremely important for the resynchronizing times to be very accurately defined, because they are less numerous than in the processes referred to hereinbefore.
However, numerous phenomena can be responsible for a deterioration in the synchronization quality. An example is constituted by focusing errors relative to the reading beam, as well as to errors in the radial following of the track respectively leading to variations in the surface of the reading spot or offcentring of said spot with respect to the mean axis of the tracks to be followed. This leads to variations in the shape of the detected signals, particularly variations in the rising and falling fronts thereof. However, in order to derive a synchronizing signal from the read signals, it is conventional practice to use one of these fronts. For example, it is possible to use a threshold logic element detecting the coincidence between the amplitude of the read signal and the said threshold. The variations in the time in which this detection occurs, due to the aforementioned parasitic phenomena, then lead to an incorrect synchronization of the reading or writing of the data.